perf: speed up job integration tests

[lox]

Summary

• reuse a prepared integration git repo template instead of rebuilding the seed repo for every tester
• batch local hook fixture commits in the integration harness
• make hook wrappers respect BUILDKITE_OVERRIDE_SELF for the ExecutorTester path so env dump uses the existing bintest mock instead of self-execing the full test binary
• add regression and perf coverage around the harness changes

Results

• mise run test -- -count=1 ./...: about 93.98s -> 80.58s on macOS
• internal/job/integration in the full suite: about 1m28.5s -> 1m16.5s
• mise exec go@1.25.9 -- go test ./internal/job/integration -count=1: about 77.2s -> 70.6s

Verification

• mise exec go@1.25.9 -- go test ./internal/job/hook ./internal/job/integration -count=1
• mise run test -- -count=1 ./...